1. Field of the Invention
The present invention relates to methods and apparatus for forming a wellbore in a well. More specifically, the invention relates to methods and apparatus for forming a wellbore by drilling with casing. More specifically still, the invention relates to drilling a well with drill bit pieces connected to concentric casing strings.
2. Description of the Related Art
In well completion operations, a wellbore is formed to access hydrocarbon-bearing formations by the use of drilling. Drilling is accomplished by utilizing a drill bit that is mounted on the end of a drill support member, commonly known as a drill string. To drill within the wellbore to a predetermined depth, the drill string is often rotated by a top drive or rotary table on a surface platform or rig, or by a downhole motor mounted towards the lower end of the drill string. After drilling to a predetermined depth, the drill string and drill bit are removed and a section of casing is lowered into the wellbore. An annular area is thus formed between the string of casing and the formation. The casing string is temporarily hung from the surface of the well. A cementing operation is then conducted in order to fill the annular area with cement. Using apparatus known in the art, the casing string is cemented into the wellbore by circulating cement into the annular area defined between the outer wall of the casing and the borehole. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
In some drilling operations, such as deepwater well completion operations, a conductor pipe is initially placed into the wellbore as a first string of casing. A conductor pipe is the largest diameter pipe that will be placed into the wellbore. The top layer of deepwater wells primarily consists of mud; therefore, the conductor pipe often may merely be pushed downward into the wellbore rather than drilled into the wellbore. To prevent the mud from filling the interior of the conductor pipe, it is necessary to jet the pipe into the ground by forcing pressurized fluid through the inner diameter of the conductor pipe concurrent with pushing the conductor pipe into the wellbore. The fluid and the mud are thus forced to flow upward outside the conductor pipe, so that the conductor pipe remains essentially hollow to receive casing strings of decreasing diameter, as described below.
It is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a first designated depth with a drill bit on a drill string. The drill string is removed. A first string of casing or conductor pipe is then run into the wellbore and set in the drilled out portion of the wellbore, and cement is circulated into the annulus behind the casing string. Next, the well is drilled to a second designated depth, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed, or “hung” off of the existing casing by the use of slips which utilize slip members and cones to wedgingly fix the new string of liner in the wellbore. The second casing string is then cemented. This process is typically repeated with additional casing strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
As more casing strings are set in the wellbore, the casing strings become progressively smaller in diameter in order to fit within the previous casing string. In a drilling operation, the drill bit for drilling to the next predetermined depth must thus become progressively smaller as the diameter of each casing string decreases in order to fit within the previous casing string. Therefore, multiple drill bits of different sizes are ordinarily necessary for drilling in well completion operations.
Well completion operations are typically accomplished using one of two methods. The first method involves first running the drill string with the drill bit attached thereto into the wellbore to concentrically drill a hole in which to set the casing string. The drill string must then be removed. Next, the casing string is run into the wellbore on a working string and set within the hole within the wellbore. These two steps are repeated as desired with progressively smaller drill bits and casing strings until the desired depth is reached. For this method, two run-ins into the wellbore are required per casing string that is set into the wellbore.
The second method of performing well completion operations involves drilling with casing, as opposed to the first method of drilling and then setting the casing. In this method, the casing string is run into the wellbore along with a drill bit for drilling the subsequent, smaller diameter hole located in the interior of the casing string. In a deepwater drilling operation, the conductor pipe includes a drill bit upon run-in of the first casing string which only operates after placement of the conductor pipe by the above described means. The drill bit is operated by concentric rotation of the drill string from the surface of the wellbore. After the conductor pipe is set into the wellbore, the first drill bit is then actuated to drill a subsequent, smaller diameter hole. The first drill bit is then retrieved from the wellbore. The second working string comprises a smaller casing string with a second drill bit in the interior of the casing string. The second drill bit is smaller than the first drill bit so that it fits within the second, smaller casing string. The second casing string is set in the hole that was drilled by the first drill bit on the previous run-in of the first casing string. The second, smaller drill bit then drills a smaller hole for the placement of the third casing upon the next run-in of the casing string. Again the drill bit is retrieved, and subsequent assemblies comprising casing strings with drill bits in the interior of the casing strings are operated until the well is completed to a desired depth. This method requires at least one run-in into the wellbore per casing string that is set into the wellbore.
Both prior art methods of well completion require several run-ins of the casing working string and/or drill string to place subsequent casing strings into the wellbore. Each run-in of the strings to set subsequent casing within the wellbore is more expensive, as labor costs and equipment costs increase upon each run-in. Accordingly, it is desirable to minimize the number of run-ins of casing working strings and/or drill strings required to set the necessary casing strings within the wellbore to the desired depth.
Furthermore, each run-in of the drill string and/or casing string requires attachment of a different size drill bit to the drill string and/or casing string. Again, this increases labor and equipment costs, as numerous drill bits must be purchased and transported and labor must be utilized to attach the drill bits of decreasing size.
Therefore, a need exists for a drilling system that can set multiple casing strings within the wellbore upon one run-in of the casing working string. Drilling with multiple casing strings temporarily attached concentrically to each other increases the amount of casing that can be set in one run-in of the casing string. Moreover, a need exists for a drill bit assembly which permits drilling with one drill bit for subsequent strings of casing of decreasing diameter. One embodiment of the drilling system of the present invention employs a drilling assembly with one drill bit comprising drill bit pieces releasably connected. Thus, one drill bit is used to drill holes of decreasing diameter within the wellbore for setting casing strings of decreasing diameter. In consequence, operating costs incurred in a well completion operation are correspondingly decreased.